A light-emitting element using an organic compound is an element in which a layer including an organic compound or an organic compound film emits light by applying an electric field. The emission mechanism is said to be as follows: when a voltage is applied to electrodes with an organic compound film interposed therebetween, an electron injected from a cathode and a hole injected from an anode are recombined in the organic compound film to form a molecular exciton, and energy is released to emit light when the molecular exciton returns to the ground state.
In such a light-emitting element, usually, an organic compound layer is formed to be a thin film approximately 1 μm or less. In addition, since such a light-emitting element is an element in which an organic compound itself emits light, a backlight as used for a conventional liquid crystal display is not necessary. Therefore, such a light-emitting element has the big advantage of being able to be manufactured to be thin and lightweight. In addition, for example, in an organic compound film on the order of 100 to 200 nm, the time from injection of carriers to recombination is approximately several tens nanoseconds in consideration of the carrier mobility of the organic compound film, and light gets to be emitted approximately within microseconds even when the process from the recombination of the carriers to light emission is included. Therefore, it is also one of features that the response speed is quite fast. Further, since such a light-emitting element is a carrier-injection light-emitting element, driving by a direct voltage is possible, and noise is not easily generated. As for the driving voltage, a sufficient luminance of 100 cd/m2 is achieved at 5.5 V when an organic compound film is a uniform thin film approximately 100 nm in thickness, an electrode material is selected so as to reduce the carrier injection barrier for the organic compound film, and further, a heterostructure (a two-layer structure here) is introduced (for example, refer to Non-Patent Reference 1).
(Non-Patent Reference 1)
    C. W. Tang, et al., Applied Physics Letters, vol. 51, No. 12, pp. 913-915 (1987)
In addition to such element characteristics such as slimness, lightweight, high-speed response, and direct-current low-voltage driving, it can be also said to be one of big advantages that the luminescent color of a light-emitting element using an organic compound is rich in variation, and the factor is the variety of organic compound themselves. Namely, the flexibility of being able to develop materials for various luminescent colors by molecular design (for example, introduction of a substituent) or the like produces richness of colors. It can be said that the biggest application field of a light-emitting element utilizing this richness of colors is a full-color flat-panel display because there are a lot of organic compounds capable of emitting light's primary colors of red, green, and blue, and thus, full-color images can be achieved easily by patterning of the organic compounds.
It can be said that the above-described element characteristics such as slimness, lightweight, high-speed response, and direct-current low-voltage driving are also appropriate characteristics for a flat-panel display. However, in recent years, the use of not fluorescent materials but phosphorescent materials has been tried as an attempt to further improve a luminous efficiency. In the case of a light-emitting element using an organic compound, luminescence is produced when a molecular exciton returns to the ground state, where the luminescence can be luminescence (fluorescence) from an excited singlet state (S*) or luminescence (phosphorescence) from an excited triplet state (T*). When a fluorescent material is used, only luminescence (fluorescence) from S* contributes.
However, it is commonly believed that the statistical generation ratio between S* and T* is S*:T*=1:3 (for example, refer to Non-Patent Reference 2). Accordingly, in the case of a light-emitting element using a fluorescent material, the theoretical limit of the internal quantum efficiency (the ratio of generated photons to injected carriers) is considered to be 25% on the ground of being S*:T*=1:3. In other words, in the case of a light-emitting element using a fluorescent material, at least 75% of injected carriers are wasted uselessly.
(Non-Patent Reference 2)
    Tetsuo TSUTSUI, Textbook for the 3rd Workshop, Division of Molecular Electronics and Bioelectronics, Japan Society of Applied Physics, p. 31 (1993)
Conversely, it is believed that the luminous efficiency is improved (simply 3 to 4 times) when luminescence from T*, that is, phosphorescence can be used. However, in the case of a commonly used organic material, luminescence (phosphorescence) from T* is not observed at room temperature, and normally, only luminescence (fluorescence) from S* is observed. This is because the ground state of an organic compound is normally a singlet ground state (S0), and thus, T*→S0 transition is a forbidden transition and S*→S0 transition is an allowed transition. In reality, in recent years, light-emitting elements in which energy (hereinafter, referred to as “triplet excitation energy”) that is emitted on returning from T* to a ground state can be converted into luminescence have been released one after another (for example, refer to Non-Patent Reference 3).
(Non-Patent Reference 3)
    Tetsuo TSUTSUI, et al., Japanese Journal of Applied Physics, vol. 38, pp. L1502-L1504 (1999)
In Non-Patent Reference 3, a metal complex including iridium as a central metal (hereinafter, referred to as “iridium complex”) is used as a luminescent material, and it can be said to be a feature that an element of the third transition series is introduced as a central metal. This metal complex is a material (hereinafter, referred to as “triplet luminescent material”) capable of converting an excited triplet state into luminescence at room temperature. As described in Non-Patent Document 3, a light-emitting element using an organic compound capable of converting triplet excitation energy into luminescence can achieve a higher internal quantum efficiency than ever before. Further, when the higher internal quantum efficiency can be achieved, the luminous efficiency (lm/W) is also improved.
However, according to the report of Non-Patent Reference 3, the half-life of luminance is approximately 170 hours when the initial luminance is controlled to be 500 cd/m2 in constant current driving, and so, a light-emitting element using a triplet luminescent material has a problem with the lifetime. On the other hand, in the case of a light-emitting element using a singlet luminescent material, the half-life of luminance reaches several to ten thousands hours when the initial luminance is controlled to be 500 cd/m2 in constant current driving, and so, it can be said that the light-emitting element has a practical use in terms of the lifetime.
Accordingly, in a light-emitting element using a triplet luminescent material, an element that can be driven for a long time is desired. This is because a light-emitting element that is high in luminous efficiency and has a long lifetime can be obtained.